The Senses Flashcards
Sensory Neurons (Afferent Neurons)
- E.g. retinal cells, olfactory epithelium cells
- Carry information from the sense organs to the brain (via an interneuron)
Stimulus acts on receptor in the neuron, causing direct depolarization
“Generator Potential” Afferent sensory neuron
Efferent Neurons
- Spinal motor neurons, pyramidal neurons, Purkinje cells of the cerebellum.
- Carry information from the nervous system to effector organs such as muscle - “Motor neuron”
Interneurons
- Have short axons
- communicate only within their immediate region
- Communicate to other neurons
Multipolar neurons
e.g. Motor Neuron
Many cellular extensions from cell body
1 axon
Most common type in vertebrates
Bipolar Neurons
2 main processes from the cell body
1 dendrite (highly branched)
Least common type in vertebrates (but is seen in retinal cells)
Unipolar Neurons
- Sensory neurons
- More common in invertebrates
- eg. environmental signals, tactile sensation
Ionotropic Transduction
A stimulus triggers channels to open by direct action.
Metabotropic Transduction
A stimulus triggers channels to open indirectly via a second messenger.
Epithelial Sensory Receptor Cell
- Stimulus acts on receptor in receptor cell, causing depolarization and release of neurotransmitter
- “Receptor potential”
Reception of Signal: Sensory Unit
- Multiple sensory cells or sensory receptors within 1 cell converge onto an afferent neuron – together this is the sensory unit
- Note different shape of sensory neuron vs motor neuron
Tonic Receptors
fire action potential as long as signal is present; slow to adapt
Phasic Receptors
do not encode duration, but may respond to beginning and end of stimulus; fast to adapt
Baroreceptors
detect pressure of vessels, the heart, digestive, reproductive and urinary tract
Proprioceptors
detect monitor position of the body
Tactile receptors
detect touch, pressure, vibrations
Preamble
Tactile receptors and accessory structures in the epidermis and dermis
- Free nerve endings (pressure)
- Merkel’s disks (pressure)
- Pacinian corpuscle (vibration)
- Ruffini corpuscle (stretch; proprioception)
- Root hair plexus (hair displacement)
Vestibular sacs/Otolith organs:
Utricle (horizontal) and Saccule (vertical)
- A membrane called the macula is composed of gelatinous matrix
- Mineralized otoliths rest on the matrix
Semi-circular canals
- No otoliths
- Hair cells protrude into a gelatinous mass called the cupula
- Detect angular acceleration and circular movement in all 3 planes
- Fluid moves stereocilia toward kinocilium = activation
- Fluid moves stereocilia away from kinocilium = inhibition
Hearing: The Cochlea
Spiral structure filled with fluids in three parallel fluid filled canals
* Cochlea is derived from the Greek word kokhlias “snail or screw”
* breaks down sound by frequency
Within the Scala media lies the Organ of Corti: Sensory organ of hearing
- Sound transmission
- Composed of Hair cells which rest on the Basilar membrane (blue) and the Tectorial membrane (purple)
- Fluid in the cochlea moves with vibrations
- Within the Scala media, this fluid vibration causes the basilar membrane to move up and down.
- Movement of the basilar membrane causes hair cells to press against the tectorial membrane and de or hyper polarize.
Outer Ear: Pinna and Auditory Canal
Extents up to Eardrum
* Visible part is called Pinna
* Sound Collection
* Sound Transformation
Inner Ear
- Cochlea (hearing)
- Vestibular sacs (equilibrium)
- Semicircular canals (equilibrium)
Ossicles - 3 Bones In The Middle Ear
Malleus: Receives vibrations from the eardrum
Incus: The middle ossicle
Stapes: Connected to the incus on one end and the oval window (inner / middle ear border) of the cochlea on the other.
Structure Of A Vertebrate Hair Cell
- Epithelial sensory cell’
- contain mechanoreceptors
- Multiple Sterocillia; actin
- One Kinocillium
Three Factors Help Discern Where A Touch Stimulus Is Coming From
- Receptive fields reveal location of stimulus
- Lateral inhibition reveals location stimulus
- Receptor Adaptation
Smell and Taste: Chemoreception
Chemical signals
* Taste (Gustation)
* Smell (Olfaction)
* Pheromones (chemicals produced by animals)
* Some ionotropic or metabotropic
Smell - Vertebrates
- Vertebrates have 1000 genes coding for odorant receptors
- Each odorant receptor can recognize multiple chemicals and become activated to varying degrees
- Vertebrates can recognize upwards to 10,000 odors
Smell - Humans
- Olfactory epithelium of mammals is in the nasal cavity
- Sensory neurons communicate to a region of the brain called the olfactory bulb
Vomeronasal Organ
- Detects pheromones (chemicals produced by animals)
- In mammals this is on the either side of the nose near the nasal septum
Invertebrate Sensilla
- Chemo- and mechanoreceptive sensory neurons 260k sensory neurons
- Some specialize in touch, others in chemoreception
- Hair-like projections of cuticle
- In the antennae – enriched in chemoreceptors
Taste
Only 5 tastes: Sweet, salty, bitter, sour and umami
* Umai (delicious) mi (essence) in Japanese and corresponds to savoury or meaty sensation
* Different kinds of taste papillae contain differing numbers of taste buds
Taste buds
- Onion shaped
- Chemicals (dissolved food) enters the pore of the taste bud – epithelial sensory cells
Cones
- Concentrated in the fovea
- Population of about 6 million
- Low sensitivity (not good in the dark)
- High acuity (very good detail in the light)
- Processes color
Rods
- Found in the periphery
- Population of about 120 million
- Higher sensitivity (better in the dark)
- Good for “global information” (big objects), poor acuity (not good for details)
- Processes black, white and shades of gray
Lateral Inhibition In The vertebrate Retina
- Horizontal cells act on bipolar cells in the outer plexiform
- Amacrine cells in the inner plexiform
- Inhibit neighbouring cells
Blind Spot
- Back of the retina where the optic nerve enters the eyeball
- located in the optic disk; lacks photoreceptive cells.
Fovea
Circular region in the middle of the eye that is purely photoreceptive cone cells.
Rod signaling pathway organization to detect light/dark
Use the Principle of Convergence:
* Many rods synapse with a single bipolar cell
* Many bipolar cells can synapse with a single Ganglion cell
– Ganglia have a large receptive field
* E.g. 100 rods to 1 Ganglion
Cone Signaling Pathway Detects Colour
- Single cone connects to a single bipolar cells which connects to a single Ganglion cell.
- Ganglion cells have a small receptive field, therefore a more detailed image is created from the cone cells.
Tools For Visual Specificity
- Differential specificity of rods (dark sensitivity) and cones (colour processing).
- Convergence in the neural code varies sensitivity.
- Organization of “receptive fields” and center surround antagonism
- Lateral Inhibition: activity of neighbouring cells influence one another
Compound Eyes
- Composed of multicellular units called ommatidia
– Each has its own lens
– Creates a mosaic image
Structure of the Ommatidium
- Cornea – protective outer layer
- Crystalline cone – lens
- Retinular cell – photoreceptive cells; have microvilli called Rhabdom
- Usually 8+ retinular cells in one ommatidium
Vertebrate Eye Cells
- Photoreceptors (rods and cones) within the retina (back of eye)
- Neurons: bipolar = vertical, Amacrine and Horizontal cells = lateral
- Ganglion Cells: Connect to the optic nerve
Differences Between Smell And Taste
Smell
* Olfactory receptor cells are Bipolar sensory neuron
* G-protein coupled receptor signaling
Taste
* Gustatory receptor cells are epithelial cells
* Variety of signaling cascades (depending on taste)
Nociceptors
- Detect harmful chemicals and stimuli (sharp objects, heat) * I.e. activated by chemical, thermal or mechanical stimuli
- Brain stimulates painful sensation
- Evolutionarilyconserved
- Nociceptors of the skin are free nerve endings
- Mechanically stimulated nociceptors have high threshold (large pressure required)
Thermoreceptors: Temperature-gated ion channels
- TRPV1, is activated at temperatures greater than 42 degrees
- Perceives pain caused by hot temperatures
- At least three types of receptors that are activated by cold:
- TRPA1 receptors respond to any temperature lower than 17oC – also pain from cold is perceived by this receptor
Ciliary photoreceptors (Vertebrates)
- Single cilium protruding from the cell, highly folded membrane
- Cilium contains photopigments
- Mammals have 2 types: Rods and Cones
- Not neurons in the classical sense (no A.P.s)
- Cones = colour/bright light
- Rods = used in dim lighting, black and white shades
Ciliary Rods and Cones
- Shape differences give their names
- Similar features:
- Outer Segment (location of photopigments)
- Inner Segments (nucleus) - Synaptic terminals
Photopigments
- G-protein coupled receptor, Opsin
- Chromophore, derivative of vitamin A - Retinal is a type of chromophore
- Chromophore absorbs light
E.g. Rhodopsin, found in rod cells
Phototransduction in Vertebrates
- Light acts on 11-cis retinal
- All-trans retinal dissociates from Opsin
– Release of chromophore is called bleaching - Opsin (G-protein coupled receptor) is now active
- Opsin activates Gi protein, Transducin
- Transducin activates phosphodiesterase (PDE) consuming cGMP, making GMP
- Reduction of 2nd messenger cGMP closes cGMP gated Na+ channel
- cells hyper polarized
